Final answer:
Gastrin is the hormone produced by G cells that stimulates the release of stomach HCl and enzyme activity, playing a key role in the digestion of proteins.
Step-by-step explanation:
The hormone produced by enteroendocrine G cells that stimulates stomach hydrochloric acid (HCl) and enzyme activity is gastrin. Gastrin plays a crucial role during the gastric phase of digestion by promoting the secretion of gastric acid by the parietal cells of the stomach mucosa. This process is essential for the proper digestion of proteins in the food we consume. When the stomach is emptied, the acidic environment is no longer necessary, at which point somatostatin is released to inhibit further secretion of hydrochloric acid, maintaining a balance within the digestive system through a negative feedback mechanism.